This invention relates to a method and apparatus for controlling a multi nozzle ink jet printhead.
There are two general types of ink jet printing, drop-on-demand (DOD) and continuous (CIJ). Drop-on-demand printing, as its name suggests, produces droplets of ink as and when required in order to print on a substrate. Continuous ink jet printing, to which the present invention relates requires a continuous stream of ink which is broken up into droplets which are then selectively charged; either charged or non-charged droplets are allowed to pass to a substrate for printing, charged droplets being deflected in an electric field either on to the substrate or into a gutter (according to design where the non-printed droplets are collected for re-use. In the first case, the droplets are deflected by an electric field onto the substrate with the uncharged drops going straight on to be collected in a gutter for re-use. The amount of charge also determines the relative printed position of the drops. In the second case, the droplets are deflected into an offset gutter, with the printing drops being the uncharged ones going straight onto the substrate. The obvious advantage of printing with the uncharged drops is that, in a multi-jet printer where several drop generators are aligned perpendicular to a moving substrate, the alignment of the drops printed on the substrate is not dependent on the ability to accurately and uniformly charge the drops. As long as the charge on the droplets is sufficient for the drops to be deflected into the gutter aperture, small variations in the charge applied will not affect the quality of the resulting print. This second type of printer is generally known as a binary jet printer as the droplets are either charged or uncharged (and do not intentionally carry varying amounts of charge that determine print position).
In typical continuous ink jet printers the printhead has a droplet generator which creates a stream of droplets of ink by applying a pressure modulation waveform to the ink in a cavity in the printhead and the continuous ink stream leaving the printhead breaks up into individual droplets accordingly. This modulation waveform is usually a sinusoidal electrical signal of fixed wavelength. The stream of ink leaving the printhead breaks up into individual drops at a distance (or time) from the printhead commonly known as the break-up point, that is dependent on a number of parameters such as ink viscosity, velocity and temperature. Provided these and other factors are kept relatively constant, then a given modulation waveform will produce a consistent break-up length. In order to induce a charge on the droplet, the charging waveform must be applied to the stream at the moment before the drop separates from the stream, and held until the drop is free (ie. must straddle the break-up point). It is therefore necessary to know the phase relationship between the modulating waveform and the actual drop separating from the stream (ie. during which part of the sinusoidal modulation waveform does break-up occur).
One method of determining this phase relationship involves a charge detector (and associated electronics), positioned somewhere after the charging electrode, which can detect which drops have been successfully charged. A half width charging pulse, progressively advanced by known intervals relative to the modulation waveform, is used to attempt to charge the droplets and the detector output analysed to determine correct charging. Because of the half width pulse, theoretically half the tests should pass and half should fail. The full width pulses used for printing would then be positioned to straddle the detected break-up point. The number of intervals that the waveform is divided into, and therefore the number of possible different phases, can vary from system to system, but usually the timing is derived from a common digital close signal, and therefore is usually a binary power (ie. could be 2, 4, 8, 16, 32 etc.). Typically, 2 and 4 intervals would not give sufficient resolution, and 32 intervals upwards would make the tests too time consuming. Using 16 intervals (ie. 16 different phases) is considered to give more than adequate accuracy without involving a detrimental number of tests.
In a multi-jet print, due to manufacturing tolerances of the nozzles and the characteristics of the (usually common) ink cavity, the break-up point for each of the streams, and therefore the phase setting for printing will be different.
Modern multi-jet printers, in order to be able to print high-quality graphics and true-type scalable fonts, utilise a large number of ink streams, placed very closely together (typically 128 jets at a spacing of 200 microns).
Although it has proved possible to manufacture charge electrodes at the required spacing, to individually charge the streams, it would not be practical to duplicate existing charge electrode driver circuitry 128 times, and so current trends lean towards the use of an integrated driver solution in which a large number of the drive circuits are implemented in one Integrated Circuit device, in order to save space, reduce power etc. With such a device, for practical reasons, it is not possible to enable, or set the level of charging voltage on an individual jet basis, and so all the high voltage drivers within the device have a common enable and common power supply.
Additionally, at present it is not possible to have a separate phase detector for each stream. The probability is that the individual detectors would never be able to isolate the charge from their own stream from the effects of any adjacent streams.
As a final handicap to existing phasing methods being applied to this type of printer, it must be noted that the xe2x80x9cnormalxe2x80x9d condition for the drop stream, ie. not printing, is for all the drops to be charged. Therefore, to test individual jets would require the detection of the non-charged state, resulting in ink being sent to the substrate. Also, the phase detector circuitry would more than likely not be able to distinguish the change in charge passing the detector when a single jet was turned off, against a background of 127 jets still on.
At different phase settings, a phase detector electrode and associated circuit are used to determine the accumulated charge on simultaneously produced droplets which have been charged by the charge electrodes and a determination is made as to whether or not the accumulated charge is above or below a reference or threshold value. If it is, then that phase is considered to have passed the test. In an ideal system, once the phase detector has been tested, the threshold value would then be standard across all printers of that design. In practice, manufacturing tolerances and varying operating conditions mean that the background noise level varies not only from machine to machine, but also during operation, and this has an effect on the charge that the phase detector xe2x80x98seesxe2x80x99 and hence on the appropriateness of the threshold level. The problem is insignificant when first phasing the jets (at start-up as described in our British patent application no. 9626706.7 and our co-pending International patent application reference MJB05643WO when only the jet under test has to be charged), but is significant when, during printing, phasing is carried out as described in our British patent application no. 9626707.5 and our co-pending International patent application reference MJB05642WO, when all non-test drops have to be charged and the noise conditions vary substantially.
Accordingly, there is a need to adjust the threshold automatically, prior to phasing the jets during printing pauses and the present invention is directed towards this requirement.
According to the present invention there is provided a method for controlling a multi-nozzle ink jet printhead having a pressure modulator for causing streams of ink emitted from the nozzles to be broken up into individual droplets, and charge electrodes and charge electrode controllers for controllably applying a charge to individual ones of the droplets in each stream, the method comprising
generating a modulation waveform to operate the pressure modulator to cause droplets to be generated in each stream;
operating the charge controllers to supply a charge signal waveform to each charge electrode; comparing the charges applied to the streams droplets to a reference or threshold value;
determining, a plurality of times, the number of droplet streams in which the droplet charges exceed the reference or threshold value and calculating an average value for the number;
repeating the step of determining, a plurality of times, the number of droplet streams in which the droplet charges exceed the reference or threshold value and calculating an average value for the number; and, if the average is less than the average previously calculated then reducing the threshold or reference value.
Preferably, if the average is less than the average previously calculated then the average is compared with a predetermined value and if the average is less than the Redetermined value then the threshold or reference value is adjusted until the average determined in the next repeated step is greater than the previous average.
The number of droplet streams determined as having droplets with a charge exceeding the threshold or reference value may be determined individually or, where groups or blocks of charge electrodes have a common charge controller, together in accordance with a determination carried out for each block or group.
The xe2x80x98phasingxe2x80x99 method is preferably carried out in accordance with the methods described in our applications referred to above.